Arthur's Seat is the 251-metre extinct volcano that sits in the middle of Edinburgh, inside Holyrood Park, and it's the best view in the city for the price of nothing. If you only have an hour, take the short route up from Dunsapie Loch. If you have half a day, start at the Palace of Holyroodhouse and do the full loop past St Anthony's Chapel. Either way, wear proper shoes: the final stretch to the summit is a scramble over bare rock, not a path.
Geologically it's what's left of a volcanic vent that was last active around 335-340 million years ago, in the early Carboniferous period: an eroded lava plug that an Ice Age glacier later scoured into the lion-like shape you can pick out from town, with a steep head at the summit and a long tail (the 'Lion's Haunch') running southeast. It sits right beside the rest of the city's big free sights in our things to do in Edinburgh hub, and it's the headline entry in both the best walks and the best views guides. This page is the deep-dive on the hill itself: every route up, the current Radical Road situation, what to wear, and when the sun actually sets.
There are really two ways up, and the choice comes down to time and how much climbing you want, not skill: the short grassy pull from Dunsapie Loch, or the classic approach from Holyrood Palace past St Margaret's Loch and Salisbury Crags. Neither is step-free, and neither is a route you'd do in flip-flops. What follows covers both routes in full, plus the ruins, the lochs, and the safety and timing details that actually change your day.